In a determination of a state parameter of an object to be monitored, there is often the question of the reliability with which the state parameter has been determined. The state parameter indicating a physical state of the object to be monitored is often determined based on a comparison of measurement data with a threshold. If, for example, the measurement data are noisy, there can be wrong decisions in the determination of the state parameter, so that a possible subsequent decision chain based on an erroneously determined state parameter is also erroneous.
If the object to be monitored is, for example, a vehicle, the state parameter can, for example, be a driving situation of the vehicle, wherein the driving situation comprises, for example, a driving state and a non-operated state.
A determination of a driving situation of a vehicle is of great significance for a plurality of security-relevant aspects. In tire pressure monitoring, for example, a driver can always be informed about the current pressure depending on the determined driving situation (state parameter), so that he can react immediately in case of a pressure drop and can stop the vehicle, for example. Normally, battery-operated pressure sensors are used for measuring and monitoring the tire pressure in a vehicle tire, which transmit their measurement values from the inside of the tire and preferably at the rim via a transmission unit to a central unit, for example a board computer, disposed outside the tire. Due to the required battery operation of the pressure sensor as well as the transmission unit, the life-span of such sensor applications is limited. This problem, however, is not only encountered with the already mentioned tire pressure sensors where a time control for measurement and transmission is performed dependent on pressure or acceleration criteria, but also with any battery-operated sensor systems, such as for separately positionable temperature measurement devices.
Due to the life-span limited by battery operation, it is important to make the decision with as few measurement cycles as possible. Since frequently decision certainties, which are to be higher than the accuracy of an individual measurement with the sensor, are required for decisions, normally, averages of a sequence of repeated measurements are used, or the measurements are taken more frequently than required and then low-pass filtered. However, both approaches increase the power consumption, in the first case in proportion to a number of measurement repetitions and in the second case in proportion to oversampling. At the same time, decisions are to be made as early as possible when the criteria are fulfilled, which results in a minimum frequency of measurements and also a minimum bandwidth of low-passes for noise filtering.
The tire pressure sensor arrangements used in the prior art for monitoring the tire pressure of a vehicle tire have a plurality of disadvantages. With the battery-operated tire pressure sensor configuration, it is not possible to take continuous measurements of the tire pressure measurement values over the whole life-span of the tire pressure sensor configuration, which is in the range of, for example, 10 years. Common tire pressure sensor configurations have too high a power consumption for that, which limits the life-span of the battery-operated tire pressure sensor configurations, so that a continuous measurement of the physical state parameters, such as pressure or temperature, can not be performed with a sufficiently high measurement repetition rate during the whole intended life-span. A sufficient measurement repetition rate is determined by the time interval during which a change of the tire pressure is to be determined, so that the shorter the time interval between the detection of the individual tire pressure measurement values and their transmission to an evaluation electronic, the higher the certainty to detect a dangerous change of the tire pressure, which indicates a critical state of the tire, soon enough.
Additionally, the power consumption of known tire pressure sensor configurations is determined by the associated sensor unit, which serves for transmitting the individual tire pressure measurement values to the central unit, which performs the further processing of the transmitted tire pressure and tire temperature values, respectively. In the so far most common tire pressure sensor configurations, the measurement frequency for detecting the tire pressure measurement values and the transfer frequency (abundance) for transmitting the tire pressure measurement values are adapted in dependence on the driving state of the vehicle detected via an additional acceleration sensor or an additional motion switch.
In order to detect pressure changes of the tire pressure, which indicate damage of the tire, as soon as possible, in the tire pressure sensor configurations known in the prior art it is further required to perform a significantly continuous measurement of the pressure and temperature values in the tire in dependence on the detected driving state of the vehicle and transmit these to the central unit across a high-frequency radio link. Thus, the relatively high turn-on frequency of the transmission unit in the known tire pressure sensor configurations leads to a relatively high average power consumption of the battery-operated configuration, which has the consequence that the intended life-span of, for example, 10 years, cannot be achieved.
WO 03/080371 A2 describes, for example, a tire pressure monitoring system, where tire pressure measurement values, which are successive in time, are detected by a transmission unit for monitoring the tire pressure in a tire of a vehicle. At least part of the tire pressure measurement values is transmitted with a variable frequency to a receiver unit, wherein the frequency is derived from the detected tire pressure measurement values via a control unit.
The above-described approach according to the prior art utilizes the fact that dynamic load redistributions occur during a driving operation of the vehicle, which leads to a change of pressure in the tires. For example in a bend, the outer wheels are more heavily loaded and consequently the pressure in these tires increases, while it decreases in the relieved wheels on the inside of the bend. The same takes place during braking or accelerating between the wheels of the rear axle and the front axle. Thereby, the transmission and measurement intervals of the tire pressure sensor system are determined by the measured tire pressure itself. For switching between a driving state and a non-operated state, switching thresholds are used, which are adapted both in the non-operated state and in the dynamical driving state, so that, for example, static pressure conditions in the tire, as they occur, for example, during parking, and dynamic driving conditions are always taken into consideration.
In the determination of the state parameter, such as the already mentioned driving situation of the vehicle, this is of particular importance for battery-operated sensor systems where the measurement values are transmitted, for example, only in the driving state. Here, a wrong decision for tranmitting measurement data causes a straining transmission process with high current consumption, while a wrong decision for not transmitting merely causes a short delay of the transmission process.
If, for example, systems are used, which additionally use an acceleration sensor to determine the driving state of the vehicle, the measurement rate and particularly the number of heavily current-consuming RF transmissions can be reduced significantly in the non-operated state compared to the driving state. However, the acceleration sensor has to be evaluated continuously in order to determine when the driving state changes and thus the measurement and transmission rates have to be changed. It applies for the frequency of checking the acceleration sensor that it has to be checked at least so frequently that a time is kept, which is available for detecting the change of the driving state.
In simple mechanical acceleration sensors where a mass is shifted against, for example, a counter-force defined by a spring by the acceleration and closes a contact at a certain point of its path, monitoring this switch represents no significant current consumption, because a signal-noise ratio of the switching signal is so high that it can be monitored continuously by a simple Schmitt trigger at the input. However, for reliability reasons, these switches are replaced by micromechanical acceleration sensors, the signals of which often have to be read out with the help of amplifiers and/or A/D converters. Here it applies that an energy consumption of the evaluation circuit increases with increasing demands on accuracy. Examples for the causes are an extension of the measurement length for measurement repetition and averaging, an increase of the sampling frequency and downstream low-pass filtering, an increase of current consumption for a more low-noise input stage or an extension of the measurement time for a measurement value with an input circuit with lower bandwidth.
If, for example, gas pressure is monitored instead of the acceleration, the measurement data is further processed and transmitted from the tire to a receiver in the vehicle. However, a large part of the data is also measured only for a derivation of decisions. For an accuracy with which this decision can be made, the same influences apply, which have been described above for the acceleration sensor.